1. Field of the Invention
The present invention relates to, for example, conversion apparatus configured to operate between a synchronous communication network and an asynchronous communication network to convert transmission signals from the communication networks in accordance with the protocols of the respective communication networks.
2. Description of the Related Art
In a conventional communication system, a synchronous communication network accommodates a transmission apparatus used for circuit switching and a terminal apparatus configured to transmit and receive signals to and from the transmission apparatus. In such a communication system, a clock is synchronized between the transmission apparatus and the terminal apparatus. The transmission apparatus and the terminal apparatus carry out data communication by transmitting and receiving data in synchronism with the clock.
In recent years, there has been a demand to change the synchronous communication network to an Internet protocol network (IP network) that is an asynchronous communication network. This is because when a communication infrastructure is used as an IP network instead of a synchronous communication network in which only special apparatuses can be connected to the network as terminals, the number of apparatuses that can be connected to the network increases sharply and apparatus costs reduces. Thus, in association with the change from a synchronous communication network to an IP network that is an asynchronous communication network, conversion apparatuses for communication between the existing synchronous communication network and the IP network have been proposed.
A known technique used for such conversion apparatuses is Circuit Emulation Service over Packet Switched Network (CESoPSN). The CESoPSN technique converts a synchronous clock into a special packet, which is then placed among IP packets for transmission. The conversion apparatus receives and converts the IP packets into original data based on the clock in the special packet.
In the IP network, IP packets are transmitted in accordance with a best-effort transmission scheme. Thus, depending on the transmission environment, transmission of the IP packets may be delayed or IP packets may be discard. Here, if the special packet included in the IP packets is discard, clock synchronization cannot be maintained between the synchronous communication network and the IP network. Thus, if an error such as a discard IP packet occurs, an alarm signal (alarm indication signal [AIS]) is provided to the transmission apparatus connected to the conversion apparatus (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 5-244203). Here, by definition, an AIS means that an error is also occurring in the clock for the circuit. Thus, upon receiving the AIS, the transmission apparatus in the synchronous communication network disconnects from the conversion apparatus. The transmission apparatus then switches the synchronization to its own clock for the synchronous communication network. The transmission apparatus waits for the IP network to recover, and upon receiving the notification that the IP network has recovered, synchronizes with the clock of the conversion apparatus. The transmission apparatus then resumes communication via the conversion apparatus in accordance with the clock.
As described above, even if an error in the IP network causes the communication to be cut off to make the clock between the synchronous communication network and the IP network asynchronous, the recovery of the IP network allows the clock to be synchronized to resume communication. Thus, the asynchronous status of the clock often fails to be acknowledged as a problem. However, if the synchronous communication network accommodates a base station configured to control communication between a plurality of radio terminals, data communication is desirably recovered quickly. In this case, after the error in the IP network is eliminated, a long time may disadvantageously be required to recover the clock synchronization between the synchronous communication network and the IP network.